1. Field of the Invention
The present invention relates to an image display, and particularly to an image display which performs color reproduction by using color filters.
2. Description of the Background Art
Recently, many displays which reproduce color images by separating light from a light source into N kinds of colors by using color filters and projecting the lights separated into N colors onto a screen are appearing. The number N is a positive integer. Usually N is equal to three, and lights separated into three colors of red, green and blue (hereinafter referred to as R, G, B) through color filters are projected to reproduce a color image.
The displays which reproduce color images by using lights color-separated by filters are roughly classified into two types according to structure. One is the time-division color projection system and the other is the simultaneous color projection system. The time-division color projection system and the simultaneous color projection system will now be described about a structure in which lights separated into the three colors R, G, B through color filters are projected to reproduce a color image.
In the time-division color projection system, lights separated into N kinds of colors through color filters are projected in order in a time-division manner within a single image frame to reproduce a color image. When reproducing a color image by projecting lights separated into the three colors R, G, B through color filters, the lights separated into R, G, and B are projected in order in a time-division manner to reproduce a color image.
In the simultaneous color projection system, lights separated into N kinds of colors through color filters are simultaneously projected to reproduce a color image. For example, when reproducing a color image by projecting lights separated into the three colors R, G, B, the lights separated into R, G, B through color filters are simultaneously projected to reproduce a color image.
FIG. 15 is a diagram showing an example of a color image display of the time-division color projection system using a light valve. The light valve is an element for modulating light, which may be a DMD, liquid crystal, etc. As shown in FIG. 15, the time-division color projection system color image display has a light source 101, a color disk 102, a light valve 103, a screen 104, and a signal processing unit 105. The color image display of FIG. 15 reproduces a color image by projecting lights separated into the three colors R, G, B.
The operation of the time-division color projection system will now be described referring to FIG. 15. Image data is inputted to the signal processing unit 105. The signal processing unit 105 generates control signals for the color disk 102 and the light valve 103 from the input image data and supplies the control signals to the color disk 102 and the light valve 103.
The light emitted from the light source 101 enters a part of the color disk 102. The color disk 102 is divided into three areas: a filter transmitting red (R) light, a filter transmitting green (G) light, and a filter transmitting blue (B) light. The color disk 102 is rotating while controlled by the control signal from the signal processing unit 105 and the filter located in the position the light from the light source 101 strikes is changed in a time-division manner. Thus the light from the light source 101 is color-separated into R, G, B lights in a time-division manner by the color disk 102. The lights color-separated into R, G, and B lights at the color disk 102 in a time division manner impinge on the light valve 103.
The R, G, and B lights incident upon the light valve 103 are modulated by the light valve 103 according to the control signal from the signal processing unit 105 and projected onto the screen 104.
Next, the simultaneous color projection system will be described. FIG. 16 is a diagram showing an example of a color image display of the simultaneous color projection system using a light valve. As shown in FIG. 16, the simultaneous color projection system color image display has light sources 101a to 101c, light valves 103a to 103c, a screen 104, a signal processing unit 105, a filter transmitting red light 106a, a filter transmitting green light 106b, and a filter transmitting blue light 106c. The color image display of FIG. 16 reproduces a color image by projecting the lights separated into the three colors R, G, B.
The operation of the simultaneous color projection system will now be described referring to FIG. 16. Image data is inputted to the signal processing unit 105. The signal processing unit 105 generates control signals for the light valves 103a to 103c from the input image data and supplies the control signals to the light valves 103a to 103c. 
The light emitted from the light source 101a enters the color filter 106a. The color filter 106a has a property of transmitting red light, and the light from the light source 101a is color-separated to R light at the color filter 106a. The R light color-separated at the color filter 106a impinges upon the light valve 103a. 
Similarly, the light from the light 101b is color-separated to G light at the color filter 106b and impinges upon the light valve 103b. The light from the light source 101c is color-separated to B light at the color filter 106c and impinges upon the light valve 103c. 
The R, G, and B lights incident upon the light valves 103a, 103b, and 103c are modulated at the light valves 103a, 103b and 103c according to the control signals from the signal processing unit 105 and projected onto the screen 104.
As described above, a display which color-separates light from light source using color filters and reproduces a color image using the color-separated lights can be realized by the time-division color projection system or the simultaneous color projection system. The two systems are common in that they color-separate the light emitted from the light source through color filters.
Now a color image display which reproduces a color image with lights generally separated into N colors will be considered. The spectral emissivity of the light emitted from the light source is shown as E(xcex) and the spectral transmittances of the N color filters for color-separating the light from the light source are shown as fi(xcex). Where i=1, 2, . . . , N. In this case, the spectral distributions ci(xcex) of the N-color lights Ci color-separated by the N color filters can be expressed by equation (1) below.
ci(xcex)=E(xcex)xc3x97fi(xcex)xe2x80x83xe2x80x83Eq.(1)
From equation (1), the spectral distributions ci(xcex) of the lights Ci separated into N colors are represented as the product of the spectral emissivity E(xcex) of the light emitted from the light source and the spectral transmittance fi(xcex) of the color filters. Accordingly, to efficiently utilize the light emitted from the light source, the characteristic of the spectral emissivity E(xcex) of the light emitted from the light source must be taken into consideration when determining the spectral transmittances fi(xcex) of the color filters.
Now suppose a light source S having such spectral emissivity as shown in FIG. 17. In FIG. 17, the vertical axis shows the relative intensity of light emitted from the light source and the horizontal axis shows the wavelength. The spectral emissivity of the light source S shown in FIG. 17 has intensive radiations in the two wavelength regions a and b. Such a spectral emissivity characteristic as has intensive radiations at particular wavelengths is shown by a super high pressure mercury lamp, a xenon lamp, etc.
Now we consider the color separation of light from a light source S through color filters. FIG. 18 is a diagram showing an example of spectral transmittance characteristic of a color filter F used in the color separation. FIG. 18 shows the spectral transmittance characteristic of the color filter F by the solid line and the spectral emissivity of the light source S by the broken line. In FIG. 18, the vertical axis shows the transmittance of the filter or the relative intensity of the light and the horizontal axis shows the wavelength.
FIG. 19 is a diagram showing the spectral distribution characteristic of the light Ci obtained when the light from the light source S is color-separated through the color filter F. FIG. 19 shows that the two intensive radiations of the spectral emissivity of the light source S are not sufficiently transmitted when the light from the light source S is color-separated by the color filter F. This means the fact that the energy of the light emitted from the light source S is not sufficiently utilized. This is because the cutoff wavelengths of the color filter F are in the vicinities of the intensive radiations in the spectral emissivity.
Next, a more specific example will be considered. Suppose that a super high pressure mercury lamp is used as the light source S and a color image is reproduced by projecting lights separated into three colors R, G, B. FIG. 20 shows an example of spectral emissivity Eu(xcex) of the super high pressure mercury lamp. In FIG. 20, the vertical axis shows the relative intensity of the light emitted from the light source and the horizontal axis shows the wavelength. FIG. 20 shows that the spectral emissivity Eu(xcex) of the super high pressure mercury lamp has intensive radiations at some wavelengths.
FIG. 21 shows an example of spectral transmittance characteristics f1i(xcex) of three color filters used in the color image display. Where i=1 to 3. In FIG. 21, the vertical axis shows the transmittance of the filters and the horizontal axis shows the wavelength. In this case, the spectral distribution c1i(xcex) of the three-color light C1i color-separated by the three color filters can be calculated by the equation (2) below.
c1i(xcex)=Eu(xcex)xc3x97f1i(xcex)xe2x80x83xe2x80x83Eq.(2)
FIG. 22 shows the spectral distributions c1i(xcex) of the three-color lights C1i color-separated by the color filters. In FIG. 22, the vertical axis shows the relative intensity of the color-separated lights, and the horizontal axis shows the wavelength. The tristimulus values X1i, Y1i, Z1i of the color-separated three-color lights Ci can be obtained by performing the calculation of equation (3) by using c1i(xcex) obtained from equation (2). In equation (2), (xcex), (xcex), (xcex) show the color matching function.
X1i=∫c1i(xcex)xc3x97{overscore (x)}(xcex)dxcex=∫Eu(xcex)xc3x97f1i(xcex)xc3x97{overscore (x)}(xcex)dxcex
Y1i=∫c1i(xcex)xc3x97{overscore (y)}(xcex)dxcex=∫Eu(xcex)xc3x97f1i(xcex)xc3x97{overscore (y)}(xcex)dxcex
Z1i=∫c1i(xcex)xc3x97{overscore (z)}(xcex)dxcex=∫Eu(xcex)xc3x97f1i(xcex)xc3x97{overscore (z)}(xcex)dxcexxe2x80x83xe2x80x83Eq.(3)
Further, with the x1i and y1i values obtained by performing the calculation of equation (4) using the tristimulus values X1i, Y1i, Z1i obtained by equation (3), the color-separated three-color lights C1i can be represented on a chromaticity diagram. The chromaticity diagram shows colors as points on plane coordinates.                               x1i          =                      X1i                          X1i              +              Y1i              +              Z1i                                      ⁢                  
                ⁢                  y1i          =                      Y1i                          X1i              +              Y1i              +              Z1i                                                          Eq        .                  (          4          )                    
FIG. 23 shows the three-color lights C1i color-separated through the color filters on a chromaticity diagram. In FIG. 23, the inside of the triangle defined by the three points C11, C12, C13 corresponds to the coordinates of colors reproducible with the three-color lights C1i. Thus, in a color image display using a super high pressure mercury lamp as a light source and performing color separation with three color filters having the spectral transmittance characteristics shown in FIG. 21, the range of reproducible colors corresponds to the inside of the triangle defined by the three points C11, C12, C13 on FIG. 23.
Recently, multi-band image displays are studied in which a color image is reproduced by using lights separated into more than three colors. The multi-band image display is advantageous in that the range of reproducible colors can be expanded as compared with a conventional three-color image display.
Reproduction of a color image with lights separated into six colors will now be considered as an example of the multi-band image display. Six color filters used to color-separate the light from a light source may have such spectral transmittance characteristics as almost equally divide the wavelength region of 400 to 700 nm into six. FIG. 24 shows an example of the spectral transmittance characteristics f2j(xcex) of the six color filters. Where j=1 to 6. In FIG. 24, the vertical axis shows the transmittance of the filters and the horizontal axis shows the wavelength.
Suppose that a super high pressure mercury lamp is used again as the light source. FIG. 25 shows the spectral distributions c2j(xcex) of the six-color lights C2j color-separated by the color filters. The spectral distributions c2j(xcex) can be given by the calculation of equation (5). In FIG. 25, the vertical axis shows the relative intensity of the color-separated lights and the horizontal axis shows the wavelength.
c2j(xcex)=Eu(xcex)xc3x97f2j(xcex) xe2x80x83xe2x80x83Eq.(5)
The tristimulus values X2j, Y2j, Z2j of the color-separated six-color lights C2j can be obtained by performing the calculation of equation (6) by using c2j(xcex). Further, with the x2j and y2j values obtained by performing the calculation of equation (7) by using the tristimulus values X2j, Y2j, Z2j, the color-separated six-color lights C2j can be represented on a chromaticity diagram.
X2j=∫c2j(xcex)xc3x97{overscore (x)}(xcex)dxcex=∫Eu(xcex)xc3x97f2j(xcex)xc3x97{overscore (x)}(xcex)dxcex
Y2j=∫c2j(xcex)xc3x97{overscore (y)}(xcex)dxcex=∫Eu(xcex)xc3x97f2j(xcex)xc3x97{overscore (y)}(xcex)dxcex
Z2j=∫c2j(xcex)xc3x97{overscore (z)}(xcex)dxcex=∫Eu(xcex)xc3x97f2j(xcex)xc3x97{overscore (z)}(xcex)dxcexxe2x80x83xe2x80x83Eq.(6)
                              x2j          =                      X2j                          X2j              +              Y2j              +              Z2j                                      ⁢                  
                ⁢                  y2j          =                      Y2j                          X2j              +              Y2j              +              Z2j                                                          Eq        .                  (          7          )                    
FIG. 26 represents the six-color lights C2j color-separated by the color filters on a chromaticity diagram. In FIG. 26, the interior of the hexagon defined by the six points C21 to C26 correspond to the coordinates of colors reproducible by the six-color lights C2j. Accordingly, in a color image display using a super high pressure mercury lamp as a light source and performing color separation with six color filters having the spectral transmittance characteristics shown in FIG. 24, the range of reproducible colors corresponds to the interior of the hexagon defined by the six points C21 to C26 in FIG. 26.
The triangle shown by the broken line in FIG. 26 is the range of colors reproducible in the color image display using a super high pressure mercury lamp as a light source and performing color separation with three color filters having the spectral transmittance characteristics shown in FIG. 21. FIG. 26 shows that the color image display using six color filters having the spectral transmittance characteristics shown in FIG. 24 can reproduce a wider range of colors as compared with the color image display using three color filters having the spectral transmittance characteristics shown in FIG. 21.
Now we consider the relation between the spectral transmittance characteristics of the six color filters shown in FIG. 24 and the spectral emissivity characteristic of the super high pressure mercury lamp shown in FIG. 20. In the spectral transmittance characteristics of the six color filters, f23(xcex) and f24(xcex) have cutoff wavelengths in the vicinity of the wavelength 550 nm. On the other hand, the spectral emissivity characteristic of the super high pressure mercury lamp has intensive radiation in the vicinity of the wavelength 550 nm.
From this relation, the intensive radiation around the wavelength 550 nm of the spectral emissivity characteristic of the super high pressure mercury lamp is largely cut by the color filters. Accordingly it cannot be said that the six color filters having the spectral transmittance characteristics shown in FIG. 24 effectively utilize the light energy emitted from the super high pressure mercury lamp having the spectral emissivity characteristic shown in FIG. 20.
Further, the spectral transmittance characteristics of the color filters sharply change in the vicinities of the cutoff wavelengths. Hence, when the cutoff wavelength of the spectral transmittance characteristic of a color filter is in a wavelength region in which the spectral emissivity characteristic of the light source presents intensive radiation, a slight shift of the spectral transmittance characteristic of the color filter appears as a significant difference in the spectral distribution of the color-separated light.
As stated above, depending on the relation between the spectral emissivity characteristic of the light from the light source and the spectral transmittance characteristics of color filters used for color separation, a display reproducing a color image by using lights color-separated by the color filters cannot sufficiently utilize the light energy from the light source, and a slight shift of the spectral transmittance characteristics of the color filters appears as a significant difference in the spectral distribution of the lights after color-separated.
This problem is especially serious in a display which reproduces a color image using lights color-separated into four or more colors through color filters.
A conventional image display reproducing a color image by using lights color-separated by color filters into four or more colors has the problem that it cannot sufficiently utilize the energy of light from the light source used therein, depending on the relation between the spectral emissivity characteristic of the light from the light source and the spectral transmittance characteristics of the color filters used for color separation. It also has the problem that a slight shift of the spectral transmittance characteristics of the color filters appears as a significant difference in the spectral distributions of the lights after color-separated, depending on the relation between the spectral emissivity characteristic of the light from the light source used therein and the spectral transmittance characteristics of the color filters used for color separation.
A first aspect of the present invention is directed to an image display having a light source and a color filter and color-separating light emitted from the light source through the color filter to reproduce a color image by using the color-separated light, wherein the color filter has its cutoff wavelength in a wavelength region in which a spectral distribution obtained by multiplying the spectral emissivity characteristic of the light emitted from the light source by a color matching function is smaller than a predetermined threshold, which enables effective use of the energy of the light emitted from the light source and alleviates the effect that a slight shift of the spectral transmittance characteristic of the color filter exerts on the spectral distribution of the light after color separation.
Preferably, in an image display of a second aspect, the color filter includes four or more kinds of color filters, and then the range of reproducible colors can be expanded while successfully solving the problems which become more serious as the number of kinds of the color filters increases, that is, the problem that the energy of the light from the light source cannot sufficiently be utilized and the problem that a slight shift of the spectral transmittance characteristics of the color filters appears as a significant difference in the spectral distributions of the lights after color separated.
Preferably, in an image display of a third aspect, the predetermined threshold is a value obtained by multiplying an average value of the spectral distribution in a predetermined wavelength region by a constant, and which is smaller than the maximum value of the spectral distribution in the predetermined wavelength region, and then the cutoff wavelengths can be selected in accordance with the number of kinds of the color filters by appropriately setting the constant, so that the energy of the light emitted from the light source can be utilized further effectively and the effect exerted by a slight shift of the spectral transmittance characteristic of the color filter on the spectral distribution of the light after color-separated can be further reduced.
Preferably, in an image display of a fourth aspect, the predetermined wavelength region is the wavelength region from 380 nm to 780 nm, and the cutoff wavelength can be selected in accordance with the wavelength region of 380 to 780 nm which corresponds to the visible region of human eyes, so that the energy of the light emitted from the light source can be utilized further effectively and the effect of a slight shift of the spectral transmittance characteristic of the color filter on the spectral distribution of the light after color separation can be further alleviated.
Preferably, in an image display of a fifth aspect, the constant is an integer, which facilitates the selection of the cutoff wavelength.
Preferably, in an image display of a sixth aspect, the light source is a super high pressure mercury lamp, and in an image display using a super high pressure mercury lamp as the light source, the energy of the light emitted from the super high pressure mercury lamp can be effectively used and the effect of a slight shift of the spectral transmittance characteristic of the color filter on the spectral distribution of the light after color separation can be reduced.
Preferably, in an image display of a seventh aspect, the light source is a metal halide lamp, and in an image display using a metal halide lamp as the light source, the energy of the light emitted from the metal halide lamp can be effectively used and the effect of a slight shift of the spectral transmittance characteristic of the color filter on the spectral distribution of the light after color separation can be reduced.
The present invention has been made to solve the above-described problems, and in a display for reproducing a color image by using lights color-separated into four or more colors through color filters, an object of the present invention is to determine spectral transmittance characteristics of the color filters used for color separation while taking into account the spectral emissivity characteristic of the light from the light source used therein, so as to effectively use the energy of the light from the light source and to alleviate the effect that a slight shift of the spectral transmittance characteristics of the color filters exerts on the spectral distributions of the lights after color separation.
These and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.